1. Field of the Invention
The present invention concerns a device for cooling a rolled ferrous or non-ferrous product, especially a steel strip.
2. Description of the Prior Art
Heat treatment of rolled products that pass vertically over rollers and through successive treatment chambers is known in itself. In the manufacture of steel plate for automobile bodies, continuous annealing or galvanization lines are used on which the steel is heated to temperatures of up to 600.degree. C.-900.degree. C. Rapid and uniform cooling of these products is then needed to reduce the temperature of the product to a temperature below 500.degree. C. depending on the quality required.
Various cooling methods have been used before now. Passing the rolled product over cooled rollers or immersing it in a liquid or a semi-liquid medium is known in itself, for example. These two-phase conduction or convection cooling methods provide local thermal transfer coefficients in excess of 400 kCal/m.sup.2 .multidot.h.multidot..degree.C., but for small temperature drops. Moreover, these methods have the drawback of generating problems of oxidation of the rolled product and contact of the rolled product with the cooling liquid or solid frequently causes flatness defects.
Another type of method, by spraying a gas, avoids the previously mentioned drawbacks. U.S. Pat. No. 4,363,471 describes a steel annealing line in which the steel strip passes across the front of a box containing a series of gas blower nozzles. These nozzles project only slightly from the surface of the box, however. Evacuation of the gas after it impinges on the steel strip is impeded by the box: back-pressure areas then arise between the nozzles and the box, disrupting the blowing of the cooling gas towards the steel strip. Moreover, the gas can only escape laterally, across the width of the rolled product, which produces differential cooling of the edges of the rolled product and may lead to flatness defects. The thermal transfer coefficients achieved by this type of device do not exceed 200 kCal/m.sup.2 .multidot.h.multidot..degree.C. for a gas comprising a mixture of nitrogen and 5 hydrogen, and even lower for air.
In the article by T. Kaihara et al "New technology in KM-CAL for sheet gage" published in "Developments in annealing rolled steel", ed. Pradan and Gupta, 1992 it is indicated that a maximum rate of 50.degree. C./s can be obtained for a rolled product with a thickness equal to 0.7 mm, which is equivalent to a transfer coefficient of around 175 kCal/m.sup.2 .multidot.h.multidot..degree.C.
An article by Hiroshi Takechi entitled "Recent developments in the Metallurgical Technology of Continuous Annealing for Cold-rolled and Surface-coated sheet steels" in the same publication discloses that, even if the gas outlet orifices are at a distance of 50 mm from the rolled product, it is not possible to obtain a cooling rate of better than 100.degree. C./s for a plate less than 0.35 mm thick, corresponding to a transfer coefficient of 200 kCal/m.sup.2 .multidot.h.multidot..degree.C.
Document WO 92/02316 describes a cooling device in which an extrusion moves horizontally between fin-form pipes having gas outlet orifices in the transverse direction of the extrusion. Only the relative position of the top and bottom fins, in a staggered arrangement, is specified to obtain uniform cooling of the extrusion. In this document, however, there is no discussion of the problem of evacuating the gas after it impinges on the extrusion. The impingement of the gas on the extrusion is disrupted by the stagnant gases between the fins.
The article by IMOSE "Heating and cooling technology in continuous annealing" (ISIJ Transactions, Vol. 25, 1985, 911-932) indicates that a thermal transfer coefficient equal to 250 kCal/m.sup.2 .multidot.h.multidot..degree.C. at most can be obtained by increasing the speed and the volume of the gas, by reducing the distance between the rolled product and the blower nozzles and by enriching the gas with hydrogen. This value of the thermal transfer coefficient is nevertheless too low to significantly accelerate cooling of the rolled product.
Moreover, all of the methods that increase the hydrogen content in order to increase the transfer coefficient are difficult to render compatible with safety standards and represent real hazards to the operators.
The table below summarizes the various methods proposed before now for cooling a steel strip from 600.degree. C. to 400.degree. C.
______________________________________ Rate of cooling between 600.degree. C. and 400.degree. C. for a Heat transfer steel strip Cooling coefficient 1 mm thick method (kCal/m.sup.2 .multidot. h .multidot. .degree.C.) (.degree.C./s) Remarks ______________________________________ Gas jets - 100 17 too low normal 250 42 only for a high extreme hydrogen possible* content (*IMOSE) Cooled rollers 1000 160 serious flatness defects Immersion in 400 67 Oxidation of hot water product (.gtoreq.90.degree. C.) Immersion in 6000 1000 Oxidation of cold water product and impossible to stop cooling By mist spray 600 100 Oxidation of product cold water product and impossible to stop cooling By mist spray 600 100 Oxidation of product ______________________________________
The aim of the present invention is to propose a gas projection type cooling device that can cool a rolled product with a thermal transfer coefficient greater than 350 kCal/m.sup.2 .multidot.h.multidot..degree.C., using an innocuous gas.